This invention relates to electronic amplifiers; and more particularly to electronic differential amplifiers integrated on a single semiconductor chip. In the prior art, a wide variety of specific circuits have been developed for differential amplifiers. Basically, these amplifiers all have a high open loop gain and a high input impedance. Typically, the gain of the amplifier, with no external feedback is on the order of several thousand; and the input impedance is on the order of several hundred thousand ohms.
These differential amplifiers have a wide variety of applications. For example, they are used in stereo systems to amplify the signal to the speaker. They are also used in tape recording systems to drive the write head. Additionally, they are used as operational amplifiers and as function generators. These and other applications are described, for example, in The Linear Applications Handbook #2 by National Semiconductor Corporation, 1977.
In the past however, all of the differential amplifiers had only a single output. Thus, one operational amplifier was typically used for each operation or function to be performed. In other words, each of the amplifiers had one feedback network; and this feedback network was tailored to perform a single function. Alternatively in the past, multiple functions were performed by a single operational amplifier which had multiple feedback networks that were mechanically switched across the amplifier in accordance with the particular function to be performed. Simultaneously, the output of the amplifier was mechanically switched to one of several other circuits that were to receive the amplifier's output depending upon the function it performed.
Various undesirable aspects are associated with both of these prior art circuits. For example, when one differential amplifier performs only one function, then any system which performs multiple functions requires a corresponding multiple of amplifiers. As a result, these multi-function systems are expensive and awkward to package. Alternatively, a multi-function system that uses only a single differential amplifier and mechanically switched feedback circuits is also unattractive. This is because these systems require multiple switches, which also are expensive and awkward to package.
Accordingly, it is one object of the invention to provide an improved differential amplifier.
Another object of the invention is to provide a differential amplifier on a single chip having more than one output.
Still another object of the invention is to provide an amplfier on a single chip having multiple outputs which are selectively activated in response to logic signals.